It has been a common problem, especially in battlefield scenarios, to be able to detect troop movement so that one may know of the presence of hostile forces, whether they are assembled in a group or individually. Intrusion detection has been attempted by radar-type devices, acoustic devices, trip wires, video sensors, seismic sensors and thermal sensors, with these sensors designed either to detect moving vehicles, ordnances or the like, but not directed specifically to the detection of an individual per se.
The presence of individuals has been detected by trip wires that have to be positioned ahead of time, such that when an individual physically contacts the trip wire, an indication is provided of the presence of the individual. However, this type of mechanical sensor has an extremely high false alarm rate, for instance, when tree limbs or the like fall on them, animals trip over them or are activated by high winds, lightning and other anomalies.
Infrared has been used in the past to detect the presence of individuals but the use of infrared as a medium is problematic due to changes in scene illumination, such as changes that can occur when a cloud goes overhead.
Video surveillance is a known technique but requires a tremendous amount of bandwidth and a human being to review the reconstruction of the video scene to ascertain when a person is intruding. Pattern recognition has been employed when using video surveillance. However, pattern recognition requires a large amount of computer overhead, is not necessarily portable and oftentimes fails with changes in the scene, for instance, due to fluttering leaves, moving branches and the like.
Radar has also been used in attempt to detect intrusion. However, radar only reflects off of objects having a high radar cross-section and is not often used because the radar systems are active systems that reveal their existence and location. Moreover, when the radar is on, individuals know that they have been illuminated, and are thus able to avoid areas swept by radar.
What is therefore needed is a completely passive system that is extremely low power and is portable, such that the system can be deployed in a stealth fashion so that enemy troops are not aware of the presence of the intrusion sensing system. The system should be resistant to false alarms from such things as animals, falling branches and the like, and should be effective without detectable emissions, thus to provide a completely covert or stealth system.